If you're out to have fun and not be at the mercy of a competitive advantage, buy a gun you really like to shoot, and use it in whatever division it is lumped in. It isn't un-natural though, to want to excel, and you may find yourself wanting for another gun, and all the high-speed IPSC specific gear to go along with it. All the divisions are gamey, with Production being the least gamey (in respect to cost of gear and firearm modifications) IMHO.
Competing with your Kimber would put you in Standard. If you like your Kimber, go forth and shoot it.
Popular thought on a .45 is to be aiming for shooting 'major' in Standard. Minimum major Power Factor is 170, and assuming you are using 230gr. ball, you can decrease your velocity to around 740 fps. Depending on your pistol, this may be near on factory loads.
PF= Bullet Weight (grains) x Average Velocity (FPS) divided by 1000.
You could shoot a .40S&W in major, but a 9mm would bump you to minor.
Minor scoring is less adventageous in Standard division. (less score for peripheral hits on paper targets)
All scoring in Production division is scored as minor, regardless of caliber, making 9mm the prefered caliber of choice.
I'd recommend taking the BB, and start shooting with the Kimber you have.
Get a feel for the game, then go out and try a few other pistols once you have an idea of where you want to be in the sport.